1. Technical Field
The present invention relates to an optical space transfer apparatus realizing high speed optical space transfer using an image sensor.
2. Background Art
An optical space transfer system is capable of realizing high speed data transfer without using an optical fiber, and therefore is used for communication between buildings or as a part of wiring inside a building. The optical space transfer system is also capable of transferring a plurality of optical signals as being spatially separated from one another, and therefore does not need light sources or wavelength filters for different wavelengths, unlike a wavelength multiplex system used in optical fiber communication. Owing to these features, the optical space transfer system can use, for example, a laser which is not selected for any specific wavelength, an LED having a wide light emitting spectrum as a light source or the like. As a result, the optical space transfer system can realize high speed data transfer at low cost.
FIG. 13 shows a structure of a conventional optical space transfer apparatus 300 described in Patent document 1. As shown in FIG. 13, the optical space transfer apparatus 300 includes a transmission device 301 and a reception device 302. The transmission device 301 includes a serial-parallel conversion unit (hereinafter, referred to as an “S-P conversion unit”) 303 for converting input serial format data into parallel format data (hereinafter, such conversion will be referred to as “S-P conversion”) and a light emission unit 304 including a plurality of light sources. The reception device 302 includes a lens 305, a PD array unit 306 including a plurality of photodiodes (hereinafter, referred to as “PDs”) arranged in a matrix, and a parallel-serial conversion unit (hereinafter, referred to as a “P-S conversion unit”) 307 for converting input parallel format data into serial format data (hereinafter, such conversion will be referred to as “P-S conversion”).
The S-P conversion unit 303 performs S-P conversion on input transfer data. Parallel format data obtained as a result of P-S conversion is input to the light emission unit 304. Parallel format data includes a plurality of pieces of data (hereinafter, referred to as “parallel data”). The light emission unit 304 inputs each piece of parallel data to a corresponding one of the light sources and emits an optical signal from each light source. In FIG. 13, four pieces of parallel data are respectively input to four light sources, and four optical signals are emitted. The lens 305 collects the optical signals emitted from the light sources of the light emission unit 304 to the PD array unit 306. The PD array unit 306 converts the optical signal directed to irradiate each PD into an electric signal (hereinafter, such conversion will be referred to as “opto-electric conversion”). Each of the PDs of the PD array unit 306 outputs an electric signal obtained by opto-electric conversion. Thus, an output from the PD array unit 306 includes a plurality of pieces of parallel data. The P-S conversion unit 307 performs P-S conversion on the input plurality of pieces of parallel data to reproduce serial format data.
As described above, the optical space transfer apparatus 300 performs optical space transfer after converting serial format data into parallel format data, and thus realizes high speed transfer.
However, the optical space transfer apparatus 300 using the PD array has the following problems. First, the reception device 302 needs to perform P-S conversion and so is required to include the P-S conversion unit 307. As a result, the circuit scale is enlarged. Second, in the case where the number of the PDs included in the PD array unit 306 is equal to the number of the light sources included in the light emission unit 304, the light sources and the PDs need to correspond one to one. Therefore, precise optical axis adjustment is necessary.
As an apparatus for solving these problems, a conventional optical space transfer apparatus 400 shown in FIG. 14 is conceived. As shown in FIG. 14, the optical space transfer apparatus 400 includes a reception device 401 in place of the reception device 302 included in the optical space transfer apparatus 300. The reception device 401 includes an X-Y address image sensor (hereinafter, referred to simply as an “image sensor”) 402. An “X-Y address image sensor” is an image sensor of a system, by which pixels from which signals are to be read are sequentially specified by an address in an X direction and an address in a Y direction and signals at the specified pixels are sequentially read. Namely, the optical space transfer apparatus 400 includes the X-Y address image sensor 402 in place of the PD array unit 306 and the P-S conversion unit 307 included in the optical space transfer apparatus 300. In FIG. 14, the reception device 401 does not include the lens 305, but the reception device 401 may include the lens 305.
Now, an operation of the optical space transfer apparatus 400 will be described. The operation of the transmission device 301 is already described above and will not be described again. An optical signal emitted by each light source of the light emission unit 304 is directed to irradiate a pixel region of the image sensor 402 in which a plurality of pixels are arranged in a matrix. The image sensor 402 sequentially reads the signals received from the respective pixels to reproduce serial format data.
As described above, the optical space transfer apparatus 400 includes the image sensor 402 and so does not need to include the P-S conversion unit 307. Therefore, an increase of the circuit scale can be suppressed. In addition, the optical space transfer apparatus 400 can alleviate the required degree of precision of optical axis adjustment by increasing the number of pixels of the image sensor 402 irradiated with the optical signals and thus increasing the surface area of the pixel region, which is a light receiving region.
Patent document 1: Japanese Laid-Open Patent Publication No. 2001-292107
However, the above-described conventional optical space transfer apparatus 400 has the following problems. The image sensor 402 is usually used for imaging. The image sensor 402 sequentially reads a signal of each of pixels. As a result, the signals of all the pixels are read, and thus one image is read. FIG. 15 illustrates an operation of the image sensor 402 for reading a signal of each pixel. As shown in FIG. 15, the image sensor 402 includes a vertical scanning circuit 403, a horizontal scanning circuit 404, pixels #1 through #16, and an output signal line 405. The number of the pixels is merely one example. The image sensor 402 selects the pixels one by one by the vertical scanning circuit 403 and the horizontal scanning circuit 404, and sequentially reads the signals of the selected pixels to the output signal line 405. More specifically, all the pixels #1 through #16 are sequentially selected and the signals of all the pixels #1 through #16 are sequentially read to the output signal line 405. By this operation, the image sensor 402 outputs the optical signals directed to irradiate the pixel region as one image signal. Therefore, it requires a relatively long time to read the signals of all the pixels. Namely, the reading speed of the image sensor 402 included in the reception device 401 is decreased in proportion to the increase of the number of pixels. As a result, there is a problem that as the number of pixels increases, the transfer speed of the optical space transfer apparatus 400 is significantly decreased.
It is possible to increase the reading speed of the image sensor 402 by decreasing the number of pixels. However, where the surface area of each pixel is fixed, this technique reduces the surface area of the light receiving region (pixel region), which makes it difficult to adjust the optical axis. In addition, where the number of pixels is decreased while the surface area of the light receiving region (pixel region) is kept the same, the optical axis adjustment is facilitated but the surface area of each pixel is enlarged. As a result, the parasitic capacitance of each pixel increases and thus the signal reading speed per unit pixel decreases.
Accordingly, an object of the present invention is to realize, in an optical space transfer apparatus which alleviates the required degree of precision of optical axis adjustment using an image sensor, high speed communication by increasing the signal reading speed of an image sensor.